Beach-cast deposition and commercial harvesting of a non-indigenous alga, Mazzaella japonica: Implications for macrofauna communities in Baynes Sound, British Columbia

Abstract

Accumulations of beach-cast seaweeds and other debris, collectively known as wrack, are a common and ecologically important occurrence along coastal regions worldwide. As the rate of seaweed introductions continues to increase, so too does their beach-cast collection for aesthetic or commercial purposes. The ecological effects of these harvests are poorly characterized in the literature, especially for nutrient-rich regions such as the Northeast Pacific. Within the southern extremity of Baynes Sound, British Columbia, beach wrack contains an introduced species of red alga called Mazzaella japonica, which became the target of a commercial beach-cast harvest in 2007. The objectives of this research were threefold: 1) to quantify the contribution of M. japonica to wrack inputs within the harvest region; 2) to explore how wrack characteristics influence macrofauna communities; and 3) to determine if the commercial removal of beach-cast seaweeds has a detectable effect on wrack characteristics and macrofauna communities. To answer these questions we monitored a series of permanent transects at six sites across the harvest region from November 2014 until March 2015. We recorded as much as 853 kg (±173 SD) of wrack per meter of shoreline, approximately 90% of which was M. japonica. Despite the removal of 675 tonnes of beach-cast seaweeds, we found that the trends in wrack biomass were similar between both harvested and unharvested locations. Macrofauna communities differed between study sites, as well as with the age class, and depth of the wrack from which they were sampled. These findings contribute to the limited literature on beach-cast seaweed harvesting and provide an important record of M. japonica wrack inputs for future research or monitoring of this non-indigenous seaweed.